Tiny Tim, the youngest member of the Cratchit family and the one who dealt with physical handicaps and poor health, reveals himself to be the most generous and loving of all the family members in his comment.
Bob Cratchit sets the pattern when he begins the toast. "'A Merry Christmas to us all, my dears. God bless us!' Which all the family reechoed" means the echoed responses by the others would have been "God bless us!" Only Tiny Tim expands the response by adding the "every one," his emphasis that no one is to be left out.
Coming from Tiny Tim, the one who was often left out because of his delicate condition and the one who was in danger of not being present to celebrate another Christmas, his comment emphasizes the love and joy of sharing family time with others that was so lacking in Scrooge's existence. The extra two words remind every reader of the story that the blessings of Christmas are meant to be extended to all persons, every one of them, everywhere.
Tiny Tim provides the pathos, or powerful emotional appeal, that Scrooge needs to convince him to change his attitude toward Christmas and people. When the Ghost of Christmas Present takes Scrooge to the Cratchits' house, Scrooge hears Bob say that Tiny Tim hoped those who saw him at church would "remember upon Christmas Day who made lame beggars walk and blind men see." Tiny Tim, despite his disability, thinks of others and God more than Scrooge, who has every material advantage in life, does. When Tiny Tim pipes up, "God bless us every one!" his pure, unselfish spirit rings out. So far, Scrooge has cursed people with his "Humbug!" Tiny Tim's loving, joyful, and selfless spirit stands in stark contrast to Scrooge's unkind, dour, and selfish attitude.
Later, during his visit with the Ghost of Christmas Future, Scrooge hears the words, "And he took a child, and set him in the midst of them." This is the scripture passage known as the Sermon on the Child Text, where Jesus used a child to illustrate the sincerity of one's faith. When one accepts a child in Jesus's name, he accepts Jesus and God.
Dickens is using Tiny Tim as a kind of "Child Text." If and when Scrooge accepts the worth of this child—someone who offers no pecuniary benefit but only creates a monetary drain on the family—then Scrooge will have come to the place where he can "keep Christmas well." Indeed, Tiny Tim's remark creates in Scrooge "an interest he had never felt before." Within a page, he is "overcome with penitence and grief."
When Tiny Tim speaks this blessing, Scrooge's heart begins to warm toward humanity. Although Tiny Tim didn't know he was being watched by Scrooge, his blessing actually took in the old man as well, and Scrooge was blessed with a new way of seeing and feeling from that point on.